This invention is addressed to gas sensors generally and more particularly to particulate shields for oxygen sensors and to techniques for extending the useful life and for improving the efficiency of such a sensor when utilized in hostile environments, such as within the corrosive heated exhaust stream of an internal combustion engine.
It is well documented that a zirconium dioxide element when maintained at an elevated temperature can generate a voltage potential which is related to the partial pressure or concentrations of oxygen on either side of the walls of the element. More particularly, as discussed in U.S. Pat. No. 3,835,012, when a first surface of a sensor containing zirconium dioxide is exposed to a reference oxygen concentration and a second surface is exposed to an external environment having an unknown, but different, oxygen concentration, a voltage potential is generated between the first and the second surfaces. The generated voltage potential is indicative of the concentration of oxygen in the external environment surrounding the second surface of the zirconium dioxide element.
In general, the operation of this type of sensor is based upon the natural phenomena that a zirconium dioxide (zirconia) element becomes activated when heated. In particular, at elevated temperatures the zirconia element becomes conductive to oxygen ions. The oxygen ions tend to migrate through the zirconia element in the direction of the lowest concentration of oxygen wherein they become deposited on a thin layer of porous platinum lining the surfaces of the zirconia element. Platinum provides a high temperature electrical connection, as well as, acts as a catalyst to improve sensor performance.
Zirconium dioxide sensors can be inserted into the exhaust system of an internal combustion engine as disclosed by Burgett et al in U.S. Pat. No. 3,844,920. As the air/fuel ratio of the exhaust gas departs from a stoichiometric mixture, a voltage potential is generated indicative of the rich or lean oxygen content of the exhaust gas mixture.
The problems associated with the use of zirconium dioxide are numerable and are amplified when the element is used in a hostile atmosphere, such as in an automotive exhaust system. Zirconium dioxide is a fragile ceramic material and when subjected to the mechanical and thermal shock of the heated automotive exhaust system environment displays a shortened mechanical and electrical life. Mechanical strength can be enhanced by doping the zirconia element with yttrium oxides or oxides of magnesium. The effect of thermal shock can be minimized by allowing the gas to make a good thermal contact with a cooler surface, such as a protective shield, before the gas flow contacts the ceramic element. Thermodynamic design principles necessitate that there be a sufficient heat transfer capability between the gas and the protecting surface (shield) to dampen sudden changes in the gas temperature before the heated gas contacts the zirconia element thereby eliminating shock from sudden changes of the temperature of the gas. However, the protecting surface (shield) should not act as too great of a "heat sink," or the oxygen sensor will be prevented from reaching a proper operating temperature or alternatively, the time required for the sensor to reach its operating temperature will be greatly increased. Illustrative of the importance of not introducing large thermal delays into the gas measurements is that in a cold engine exhaust system environment, a thermally sluggish oxygen sensor will not be responsible to the oxygen content in the exhaust gases for an extended time after a cold start of the engine.
The reduction of the useful electrical lifetime of the oxygen sensor arises from the erosion of the external platinum coating on the zirconia element. Large scale erosion of this catalytic layer can be prevented by not allowing high velocity particulates within the gas to impinge directly upon the platinum surface. This can be effected by lowering the velocity of the gas before it flows over the zirconia element.
In addition to increasing the life of the platinum surface, it is desirable to enhance the sensitivity by using as much of the surface of the zirconia element as possible. This implies that the direction of the gas flow is preferably in a direction generally parallel to the element's central axis flowing from its tip to its base or vice versa rather than a flow pattern, which is generally perpendicular to one side of the element, as disclosed in the prior art.
Exemplary of sensors having shields which create perpendicular flow about the zirconia element in U.S. Pat. No. 3,844,820, which incorporates a plurality of vane shaped openings or U.S. Pat. No. 3,835,012 which, uses plurality of tangentially arranged perforations. A third type of protective shield having elongated flared openings is shown by Weyl et al in U.S. Pat. No. 3,960,692.
Generally, the effectiveness of the oxygen sensor is reduced by particulates which accumulate upon the zirconia element such as a carbon deposit.
It is an object of the invention to provide an improved oxygen sensor shield. It is another object of the invention to reduce the thermal shock on the ceramic zirconia element. A further object is to prevent large scale erosion of the catalytic coating on the zirconia element. Another object of this invention is to prevent the deposition of carbon and other particulates onto the sensing element.
An additional object of the invention is to improve the uniformity of heating and to increase the rate of heating of the zirconia element on an exhaust gas stream.